1. Field of the Invention
This invention generally relates to solar generated power and, more particularly to a hybrid system that combines thermal and photovoltaic for energy generation and thermal storage.
2. Description of the Related Art
FIG. 1 is a solar energy collection device using a parabolic trough (prior art). The design of Brunotte et al. [1] is one of the earliest to attempt to convert a line focus into a series of higher concentration foci. In this design, a parabolic trough is used to illuminate a series of solid compound parabolic concentration (CPC) secondaries, each conjugate to a single photovoltaic (PV) cell. The trough is tracked about a polar-aligned axis, so the range of skew angles is limited to +/−23.5° throughout the year. The CPCs have square or rectangular apertures, and may have an asymmetric acceptance angle. The acceptance angle in the transverse plane of the trough is determined by the maximum rim angle of the trough. If an asymmetric trough is used, as shown, then the CPCs can be tilted to a median angle, and the acceptance angle is only half the rim angle of the trough. In the other dimension (along the axial plane of the trough), the CPCs are required to have an acceptance angle of +/−23.5°, to maintain seasonal performance.
FIGS. 2A and 2B depict a design of tracking secondaries for conventional troughs, off-axis troughs, and Cassegrain troughs (prior art). Cooper et al. [3] and Thesan S.p.a. [4] both employ a second degree of tracking freedom. In addition to single-axis trough tracking, the secondary optics are allowed to rotate or translate to compensate for the changing incidence angle (within the axial plane of the trough). The secondaries include hollow, rotating CPCs and solid dielectric reflectors. A design with an array of hollow CPCs operates near the prime focus. Alternatively, a laterally-translated cylinder or spherical lenses may be used [4].
FIG. 3 is a partial cross-sectional view depicting reflected and transmitted wavelength bands of a M2 spectrum splitter (prior art). In principle, such a design permits ultraviolet (UV) and infrared (IR) light to be collected as thermal energy at heat receiver 1 (HR1), and visible (Vis) and near infrared (NIR) to be collected as photovoltaic energy. A Cassegrain geometry poses a new obstacle to achieve high concentration. Since Cassegrain optics have a large focal length, the solar image formed below M2 is likewise larger. The primary concentration from a Cassegrain trough is thus lower than it would be at the prime focus. In order to keep the solar image small after the Cassegrain M2, the M2 size should be very large. However, this causes M2 to cast a large shadow on M1 (the trough). Alternatively, M2 can be made very small so that there is minimum shadowing effect. However, the size of the solar image at the base of the trough becomes very large. An alternative approach would require raising the receiver (photovoltaic cells) to be closer to M2. This allows some reduction in the focal length. However, this may affect trough stability due to a raised center of gravity. Therefore, without additional concentration, this design is impractical for both concentrated photovoltaic (CPV) and concentrated solar power (CSP) purposes.
U.S. Pat. No. 5,505,789 uses a tessellating line focus with solid secondary funnels to address the above-mentioned problems associated with Cassegrain optics [5]. U.S. Pat. No. 5,505,789 discloses line-focus lenses and a line-focused PV module. The whole system is an array of linear arched Fresnel lenses with a linear PV cell receiver located along the focal line of each lens. The photovoltaic cell receiver consists of high efficiency cells interconnected in a string with a solid secondary optical element adhesive bonded to the cells. The entrance aperture of each secondary optical element is rectangular in shape and the optical secondaries are butted up against each other in a line to form a continuous entrance aperture along the focal line. In addition to providing more concentrated sunlight, the solid optical secondaries shield the cells from air, moisture, and contaminants, and to a lesser extent against radiation damage. However, since this system does not employ Cassegrain optics or an additional means of concentrating light to the PV cells, it is a low concentrated CPV system. It is not obvious that this system can be modified to use Cassegrain optics, or that the light collected in such a system can be concentrated sufficiently for PV collection, in light of all the reasons mentioned above.
Other beam splitting approaches for solar power include Imenes et al. [6], dichroic filter designs for hybrid solar energy by DeSandre et al. [7], analysis of hybrid solar energy efficiencies by Hamdy et al. [8], and designs of hybrid solar systems by Soule et al. [9, 10].
FIG. 4 is a partial cross-sectional view of a Cassegrain hybrid trough system with PV at the bottom of the trough [11] (prior art). A similar Cassegrain trough system with beam splitter, but with no concentration at PV cells in a slit at vertex of trough is described by Jian et al. [12].
It would be advantageous if a hybrid solar system using Cassegrain optics could be designed to optimally collect both thermal and PV energy.    [1] “Two-stage concentrator permitting concentration factors up to 300× with one-axis tracking”, Brunotte, M., Goetzberger, A., & Blieske, U. (Jan. 1, 1996). Solar Energy, 56, 3, 285-300.    [2] “BICON: high concentration PV using one-axis tracking and silicon concentrator cells”, Mohr, A., Roth, T., & Glunz, S. W. (Jan. 1, 2006). Progress in Photovoltaics, 14, 7, 663-674.    [3] “Theory and design of line-to-point focus solar concentrators with tracking secondary optics”, T. Cooper. G. Ambrosetti, A. Pedretti, and A. Steinfeld, Appl. Opt. vol. 52, 8586-8616 (2013).    [4] “Solar Receiver for a Solar Concentrator with a Linear Focus”, A. Balbo Divinadio and M. Palazzetti, Thesan S.p.a., US 2011/0023866, Published Feb. 3, 2011.    [5] “Line-focus photovoltaic module using solid optical secondaries for improved radiation resistance”, L. M. Fraas and M. J. Oneill, Entech Inc., U.S. Pat. No. 5,505,789, Granted Apr. 9, 1996.    [6] “Spectral beam splitting technology for increased conversion efficiency in solar concentrating systems: a review”, A. G. Imenes and D. R. Mills. Solar Energy Materials & Solar Cells. Vol. 84, pp 19-69 (2004).    [7] “Thin-film multilayer filter designs for hybrid solar energy conversion systems”, L. DeSandre, D. Y. Song, H. A. Macleod, M. R. Jacobson, and D. E. Osborn, Proceedings of the SPIE Vol. 562, pp 155-159 (1986).    [8] “Spectral selectivity applied to hybrid concentration systems”, M. A. Hamdy, F. Luttmann, D. E. Osborn, M. R. Jacobson, and H. A. Macleod, Proceedings of the SPIE Vol. 562, pp 147-154 (1986).    [9] “Efficient hybrid photovoltaic-photothermal solar conversion system with cogeneration”, D. E. Soule, E. F. Rechel, D. W. Smith, and F. A. Willis, SPIE Vol. 562, pp 166-173 (1985).    [10] “Heat-Mirror Spectral Profile Optimization for TSC Hybrid Solar Conversion”, D. E. Soule and S. E. Wood, SPIE Vol. 653, p 172-180 (1986).    [11] “Bandwidth and angle selective holographic films for solar energy applications”, C. G. Stojanoff, J. Schulat, and M. Eich, SPIE Vol. 3789, pp 38-49 (1999).    [12] “Optical modeling for a two-stage parabolic trough concentrating photovoltaic/thermal system using spectral beam splitting technology”, S. Jian, P. Hu, S. Mo, and Z. Chehn, Solar Energy Materials and Solar Cells vol. 94 168-1696 (2010).